Against the backdrop of global warming and exhaustion of fossil resources, “biofuels” obtainable from renewable resources attract attention. Isobutanol and n-butanol, as a next-generation biofuel following bioethanol, have many advantages, such as high heat content, low corrosivity, low water miscibility, capability of being mixed at high concentrations into gasoline, and high miscibility with diesel oil fuel.
Isobutanol is found as one of the by-products called fusel alcohols produced during alcoholic fermentation by yeast. Fusel alcohols are higher alcohols including isobutanol, isoamyl alcohol, active amyl alcohol, and derived from a German word “fusel” meaning “bad alcohol.” One century ago, Enrlich first advocated that fusel alcohols are derived from amino acid metabolic system. This metabolic system is called Enrlich pathway where a 2-keto acid, which is a biosynthetic and metabolic intermediate from valine, leucine, isoleucine, methionine, phenylalanine, or the like, is changed into aldehyde by 2-keto acid decarboxylase having broad substrate specificity, and then changed into an alcohol by alcohol dehydrogenase. Isobutanol derives from 2-ketoisovalerate, which is a biosynthetic and metabolic intermediate to valine (Non Patent Literature 1).
Non Patent Literature 2 describes that in alcoholic fermentation by yeast, the amount of isobutanol (fusel alcohol) depends on proliferation of yeast.
Examples of known isobutanol producing technologies with use of recombinant bacteria include the following. For example, Patent Literature 1 discloses an isobutanol producing technology in which an acetohydroxy acid synthase gene derived from Klebsiella pneumoniae or Bacillus subtilis; an acetohydroxy acid isomeroreductase gene derived from Bacillus subtilis, Escherichia coli, or Saccharomyces cerevisiae; a dihydroxy acid dehydratase gene derived from Escherichia coli or Saccharomyces cerevisiae; a 2-keto acid decarboxylase gene derived from Lactococcus lactis; and an alcohol dehydrogenase gene derived from Escherichia coli or Clostridium acetobutylicum are expressed with use of a host, such as Escherichia coli, Saccharomyces cerevisiae, Bacillus subtilis, Lactococcus plantarum, or Enterococcus faecalis. 
However, the technologies described in the literature do not include a butanol producing technology with use of Corynebacterium glutamicum, which is a specific species of coryneform bacteria, as a host. The above-described various kinds of genes to be introduced do not include any genes derived from Corynebacterium glutamicum, which is the host of the present invention, and the literature does not describe that high expression of the endogenous genes provides a transformant excellent in capability of isobutanol production. Further, while the above-mentioned technologies are all isobutanol producing technologies involving proliferation of the microorganism to be used, the present invention proposes that a production procedure under reducing conditions involving no substantial proliferation is superior to a procedure involving proliferation (see Comparative Examples below).
Also, Patent literature 2 and Non Patent Literature 3 disclose isobutanol producing technologies in which an acetohydroxy acid synthase gene derived from Escherichia coli or Bacillus subtilis; an acetohydroxy acid isomeroreductase gene derived from Escherichia coli; a dihydroxy acid dehydratase gene derived from Escherichia coli; a 2-keto acid decarboxylase gene derived from Lactococcus lactis, Saccharomyces cerevisiae, or Clostridium acetobutylicum; and an alcohol dehydrogenase gene derived from Saccharomyces cerevisiae are expressed with use of Escherichia coli as a host.
However, what is disclosed therein does not include any butanol producing technologies in which a microorganism species, Corynebacterium glutamicum, is used as a host. Further, the isobutanol producing technologies disclosed therein are inefficient in isobutanol production.
The inventors have already disclosed a technology in which a recombinant Corynebacterium glutamicum is made to react in a reaction mixture under reducing conditions without substantial proliferation, for highly efficient production of lactic acid, succinic acid, or ethanol (Patent Literature 3). However, nothing is mentioned regarding isobutanol, whose synthetic and metabolic pathway for production differs from those of the organic compounds described in Patent Literature 3. Further, there is no teaching regarding that highly efficient production of isobutanol can be achieved without proliferation or that such a production process without proliferation is more efficient than the process involving proliferation.